This invention relates to helical traveling wave tubes, useful in amplifying RF signals in communications, data transmission, broadcasting, satellite and radar mapping applications. A novel geometry eliminates destructive interference within the tube, and results in significantly improved efficiency.
A traveling wave tube (TWT) is a device used to amplify an RF signal in a high vacuum environment. The RF signal is amplified by the interaction of the RF wave with a beam of electrons at high voltage. The electrons are emitted from an electron gun, a thermionic emitter of electrons, using a heater to achieve required temperatures, up to 1000xc2x0 C. or more. The RF signal is typically in the range of 500 MHz to 40 GHz. A traveling wave tube used to accomplish this amplification may be of either the close-coupled cavity type or the helical type. The helical type has been favored because of its simpler construction, lower cost and large band width. Both types of amplifier, however, suffer low electronic efficiency. Other disadvantages follow from high skin effect losses, resulting in part from high helix temperatures. This typically translates into a need for greater heat transfer. High temperatures also create higher I2R losses in the helix itself, due to the simple fact that electrical resistance increases with temperature.
The need for improvement in helical tubes has been recognized and many suggestions have been made over the years. Instead of ordinary helical sections, shaped conical sections have been proposed. Varying and reducing the pitch between repeating elements of the helix have been suggested. One improvement by the inventor of the present invention, U.S. Pat. No. 4,564,787, and incorporated here by reference, involved a dynamic velocity taper, varying the pitch of the helix at an exponential rate, while keeping the helix radius constant. Many traveling wave tubes include at least one sever, generally in the center of the helix. The sever acts as a sort of isolation transformer, helping prevent backward oscillations of RF waves and preventing fluctuations in the amplifier gain. While some of these solutions have improved the situation, the state of traveling wave tubes is such that electronic conversion efficiencies still remain in the range of 10 to 25%. Overall maximum efficiencies, including significant improvements by use of a multistage depressed collector, are in the range of 40-70%.
The need for improvement is not limited merely to increasing efficiency. Heat generated by each inefficiency must be removed in order to preserve structural integrity and to minimize 12R losses. Thus, metallic heat sinks or other means of removing heat have been proposed, as have a variety of other heat-transfer devices. Manufacturers of tubes have resorted to ceramics and other materials that conduct heat but do not conduct electricity, to transfer heat from the helix itself to an outside housing and from there to outside the traveling wave tube system. These materials remain expensive and difficult to manufacture, and the problem of removing heat from the helical structure remains. What is needed is a helical traveling wave tube with inherently greater efficiency; also needed is a better means of removing the heat that is generated, minimizing losses in both the RF and the electron beam portions.
A key to increasing efficiency in a traveling wave tube is to recognize the importance of the interaction between the electron beam and the RF signal. The reason that traveling wave tubes are sometimes called xe2x80x9cslow wave structuresxe2x80x9d is that the RF signal is traveling much faster than the generated electron beam, and the RF signal must be slowed down for interaction with, and amplification by, the electron beam. The formation of a helical path is the first step in the slowing process and is recognized as a means of lengthening the path. In one embodiment of the invention, a helical path of varying radius is used in conjunction with a helical structure of simultaneously varying pitch, forming an adverse space harmonics taper (ASHT) in part of the helix. It has been discovered that such a structure is capable of achieving far greater interaction between the RF signal and the electron beam, and thus achieving greater electronic efficiency in the amplification, and greater efficiency overall in the performance of a traveling wave tube.
One embodiment of the invention is a helical traveling wave tube, which includes a helical conductor with an RF input and an RF output, and an electron gun positioned concentrically with respect to the helical conductor. The electron gun consists of a negatively-biased cathode and a grounded anode, both at a near end of the helical conductor. There may also be a control grid downstream of the anode, still at the near end, and a collector at the far end of the helical conductor. The electron gun may be run in a DC mode or may be pulsed as desired through the cathode or the grid. A series of magnets surrounds the outside of the helical tube, for a magnetic field to focus the beam of electrons passing from the cathode to the collector. At least the portion of the apparatus comprising the electron gun, the helical conductor, and the RF input and output should be operated in a hard vacuum. The helical conductor has an input section corresponding to an RF input and an output section corresponding to an RF output. In a preferred embodiment, one end of the helix, the end near the RF input, is constructed with a taper in which the radius of the helix gradually decreases at the same time that the pitch of the helix decreases, where the pitch is the distance between the turns of the helix at the same angular point. It is not necessary that this taper continue for a great length. A satisfactory adverse space harmonics taper (ASHT) can be obtained with as few as three to five turns in the input section of the helical traveling wave tube to be effective. In a preferred embodiment, a dynamic velocity taper, in which the helical conductor has a constant radius and an exponentially varying pitch, may be placed near the output section of the helical conductor.